GEOGRAPHICAL DISTRIBUTION 367 



various species to the isotherms. The surface waters of the Oceans 

 are divided into zones according to the courses of the 1 0° , 1 5 ° , 20° and 

 25° C. isotheres. The great majority of algal species are confined to 

 only one zone, a considerable number occur in two, only a small 

 number occur in three zones, whilst the number extending over 

 four or five zones are very few indeed and their distribution is 

 usually by no means certain. In New England many of the species 

 are apparently separated by the 20° C. isothere which approxi- 

 mates closely to the position of Cape Cod, so that the flora to the 

 north of the Cape is essentially different to that of the south. 

 Those species Hmited to one zone are called stenothermal whilst the 

 wide ranging forms are termed eurythermal. The former species are 

 particularly characteristic of the warmer waters, but, even so, 

 many apparent eurythermal species are found on examination to be 

 essentially stenothermal. Monostroma Grevillei and Polysiphonia 

 urceolata are summer annuals in the cold waters of Greenland, but 

 in the southern part of their range they develop in winter and early 

 spring when the temperature will be the same as it is in the Green- 

 land summer. With the exception, then, of the temperatures 

 endured by the resting spores they are essentially stenothermal. 

 Ascophyllum nodosum, with a temperature range from o to 10° C, is 

 another case and in the southern part of its range the plants pass 

 into a heat rigor during the hotter months. 



Feldmann (1937) has recently drawn attention to the pheno- 

 menon of seasonal alternation of generations and seasonal dimor- 

 phism. In Ceramium corticatulum the tetrasporic plants exist only 

 at the end of autumn or in the winter whilst the sexual plants are to 

 be found at the end of summer. This is an example of seasonal 

 alternation of generations in which there are ephemeral summer 

 haploid plants with the diploid plants occurring during the winter 

 and persisting over a longer period. Seasonal dimorphism is 

 exhibited in the Mediterranean by Cutleria multifida and C. 

 monoica with their sporophytes Aglaozonia parvula and A. chilosa. 

 The two species are almost indistinguishable morphologically, but 

 the former occurs in spring in shallow waters off-shore whilst the 

 latter occurs in summer at greater depths. Another example of 

 seasonal dimorphism is shown by the two morphologically similar 

 species Polysiphonia sertularioides and P. tenerrima, the former 

 occurring on exposed rocks from December to May whilst the 



